Method of dewatering and dressing very fine-grained mineral substances



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METHOD OF DEWATERING AND DRESSING VERY FINE-GRAINED MINERAL SUB- STANCES Wilhelm Reerink, Ess'en-Bredeney, Walter Miischenborn, Essen, andErich Niitzold, Essen-Haarzopf, Germany No Drawing. Application January12, 1954,

Serial No. 403,668

Claims priority, application Germany January 14, 1953 11 Claims. (Cl.209-12) The invention relates to a method of dewatering and dressingvery fine-grained mineral substances, such as coal slurries, small oresand the like, namely such raW materials themselves and in particularconcentrates, existing in the form of veryfine grains, from a previousdress- ,ing treatment, said method not only permitting particularlysimple and intensive dewatering of such substances but also beingcapable of combination in an advantageous manner with -a dressingtreatment or further dressing treatment of such substances.

The dewatering of very fine-grained mineral substances is known to giverise to extremely great difliculties both because of the smallquantitative output of known apparatus and also because of the degree ofdewatering obtainable with the aid of such apparatus.

Thus, for example, in the dewatering of coal flotation concentrates onfilters, with the use of very fine screens or filter cloth tensions witha correspondingly low output, a reduction of the Water content to atbest 18-25 can be obtained. i

The difiiculties of dewatering are increased as the grain size of thematerial falls, particularly when very fine debris particles (clays) arestill contained therein.

In the dewatering of crude slurries, which always contain such veryfinedebris particles, the output and. result of the filtration are so poorthat mechanical dewatering of such slurries in this manner is ofdoubtful success.

.When centrifuges are used it is true that a considerable reduction ofthe water content is possible, but considerable losses of solids withthe water passing through-the screen linings of the centrifuges must befaced. These losses of solids increase as the wear on the screensincreases and, as the latter always wear very rapidly, the losses mayamount to as much as 400 g. per litre. Since such losses cannot betolerated, the very fine slurries centrifuged through the screens mustbe further subjected to after-treatment by concentration or in cyclones,and then recycled to the centrifuge. Nevertheless, the very fine solidsgradually accumulate in the water and must be removed by other means,namely by still more difficult filtering.

The invention renders it possible to effect very substantial dewateringof very fine-grained materials, such as slurries and flotationconcentrates, in screening centrifuges, while avoiding thesedifiiculties, and possibly at the same time to purify the concentratesstill further. According to the invention this is'achieved by subjectingthe material, before it is charged to the screening centrifuge (basketcentrifuge or thrust centrifuge), to previous treatment with substancesby which the coal particles are coated with an envelope rendering themhydrophobic.

It has already been proposed to subject very finegrainedvmineralsubstances to treatment with oil, acconr panied by dispersion in Water,and then to pass them to a screening centrifuge. This proposalconstitutes primarily a dressing method which aims at bringing thedebris constituents into theform of a suspension and then in the of thevery fine grains through the centrifugal screens 1s screening centrifugeseparating into coal or concentrate 2,781,904 k Patented Feb. 19, 1957ing the material contained in the matter passing through the screen ofthe centrifuge, it being thus not necessary to pay any attention to thegreatest possible avoidance of losses in the matter passing through thescreen. For this reason, the pre-treatment serving to render theparticles hydrophobic can be carried out with the aid of substan* tiallysmaller amounts of oils, namely oils of any type, even those of lowviscosity, or other suitable reagents having the efiect of rendering theparticles hydrophobic, such as hydrocarbons, Xanthates, and the like,while it is possible to absorb without difliculty fluctuations both inrespect of the amounts and in respect of the distribution of the grainsizes and of the solid content of the slurry treated.

The particles can be rendered hydrophobic in fundamentally similarmanner to that in the known method of dressing, by treating the materialwith a relatively small amount of substances rendering it hydrophobic,While it passes through a baflle-plate mill, or machines having asimilar action, such as kneading mechanisms,

condition as possible, with a high solid content.

Through the preceding hydrophobisation, the passage on the one handavoided to a great extent, being reduced to a fraction, while on theother hand the moisture content of the centrifuged material can bereduced still further with a high output, namely in certaincircumstances to values such as hitherto could not be achieved.

When the material to be dewatered is already in the form of a pureconcentrate, for example one coming from a precedingflotation dressingtreatment, its ash content on passingthrough the centrifuge is furtherlowered because the non-hydrophobic and generally highly disperseddebris fragments which are always still present even in pureconcentrates, even if in small amounts, pass through the centrifugescreens to a considerable extent-together with the water.

In the material passing through the centrifuge the hydrophobised coal orore particles contained therein will generally spontaneously float tothe surface and can be removed in the simplest manner and recharged tothe centrifuge, while the residual water can be discharged.

In the case of substances which do not float of their own accord afterpassing through the centrifuge, as a further development of the newprocess the material passing through the centrifuge can be subjected toflotation antenna;

crude slurries, flotation treatment is inserted between thehydrophobising treatment carried out in 'the same manner as describedabove and thecentrifugal treatment. It is then found particularlyadvantageous that the pretreated material, including substances whichcan otherwise be floated only poorly or not at all, is readily floatablein the hydrophobised condition thus attained.

In fundamental contradistinction to the earlier, previously mentionedmethod of dressing, however, here also the centrifugal treatment servessubstantially only for dewatering, by which all the debris particles,which are of.course also dispersed in the water, are likewise separated,while the separation of the major part of the debris is effectedpreviously, namely in the flotation.

in these circumstances, in this case the .hydrophobising treatment canbe transferred to the flotationplant itself, i. e. be carried outtogether with the flotation treatment.

The effect of the latter is that the new process is merely used for theloss-free dewatering of normal flotation concentrates, i. e. followsknown flotation plants instead .of the otherwise necessary filter andsimilar dewatering plants.

The fundamental advantage of the new process from the point of view ofdressing, in relation to the process previously proposed by us, is basedon the fact that the very fine-grained concentrate lossses contained inthe filtrate water of the centrifuge are directly reconcentrated andreturned to the dewatering stage, and thus cannot accumulate in thewater. The highly dispersed very fine debris contained in theconcentrate is separated therefrom through the centrifugal treatment andtherefore cannot render ditiicult the dewatering process as in normaldewatering by means of filters. On repetition of the centrifugaltreatment they are discharged to the waste water in the flotation cell.

A further considerable simplificationof the method described is achievedif, according to a furtherfeature of the invention, the hydrophobisingtreatment is transferred to the centrifuge itself, the substances tobedressed being charged directly to the centrifuge together with the oilsand, if required, reagents.

The wetting of the particles is then effected through the turbulence inthe admission guide mechanism or on passing through the narrow admissionslots, or on the passing of the very fine particles through the screenapertures in the presence of very finely divided 'hydrophobisingsubstances. The centrifuge losses are reconcentrated in the mannerdescribed above, for example byflotation, and recentrifuged.

This form of carrying out the method is, however, not applicable in allcases but only where particularly favourable conditionsrexist in respectof the ability of the concentrate particles to be rendered hydrophobic.

In similar manner, fine-grained concentrates from other concentrationprocesses, such as'concentrates produced by dressing on concentratingtables by means of cyclones or the like, can be dewatered.

Some examples of the new method are givenbelow:

Example I A coal flotation concentrate, whiehafter the material chargedhad first been rendered hydrophobic was produced with an ash contentof12%, was charged to'the screening centrifuge.

The dewatcrcd concentrate produced in the screening centrifuge contained6.5% of ash and 9%"of water. "The matterpassing through the centrifugehad-an ash content of 36% and was returned to the flotation treatmentfor redressing. The material leaving the flotation process contained 75%of ash.

Example II An anthracitic fine coal of a grain sizenbelow 1 mm, which initself is not fioatable, was passed with a small percentage of fueloilper ton, with the addition of. soda 4 solution, through abaffle-plate mill and thereupon charged to a stirringmechanism'fiota'tion apparatus, in which pure debris containing over 80%of ash was discharged.

The concentrate of this flotation was charged to the screeningcentrifuge with a solid content of 450 g. per litre.

Awater content of the dewatered material amounting to 11% was obtained,while the material'passing through the centrifuge, which consisted ofcoal and very fine debris, contained 35% of ash andthe slurry had asolid content of 40 g. per litre. This material leaving the centrifugewas returned to the fiotation.

Example III A crude ore having a sulphur content of 70 to was passedwith flotation oil through'a baffle-plate mill and simultaneously groundand hydrophobised in the latter.

- The material treated in this manner was separated in a flotation plantinto a foam having a high solid content and debris and the foam waspassed to a screening centrifuge, in which it was dewatered to a watercontent of 9%. The relatively pure matter passing through the screeningcentrifuge was returned to the flotation. The concentrate obtained had asulphur content of 98.6%.

We claim:

1. A method of dewatering and dressing very finegrained substances whichcomprises in combination the steps of hydrophobising said substances,continuously centrifugally screening the hydrophobised substances,separating the matter passing through the centrifugal screen intofloatable particles and Waste water, and returning the floating fractionto the continuous centrifugal screening.

2. A method of dewatering and dressing very fincgrained substances whichcomprises in combination the steps of subjecting said substances totreatment with oilcontaining substances to render them hydrophobic,continuously centrifugally screening the hydrophobised sub stances,separating the matter passing through the centrifugal screen intofloatable particles and waste Water, and returning the floating fractionto the continuous centrifugal screening.

ing the matter passing through the centrifugal screen into floatableparticles and waste water, and returning the floating fraction to thecontinuous centrifugal screening.

5. A method of dewatering and dressing very fine ores which comprises incombination the steps ofhydrophobising said ores, continuouslycentrifugally screening the hydrophobised ores, separating the matterpassing through the; centrifugal screen into floatable particles andwaste water, and returning the floating fraction to the continuouscentrifugal screening.

6. A method of dewatering and dressing veryfinegrained substances whichcomprises in combination the steps of hydrophobising said substances,continuously centrifugally screening the hydrophobised substances,subjecting the matter passing through the centrifugal screen toflotation treatment, and returningthe concentrate from said flotationtreatment to the continuous centrifugal screening.

7. A method of dewatering and dressing very finegrained substances whichcomprises in combination the steps of hydrophobising said substances,subjecting said hydrophobised substances to flotation treatment,subjecting the concentrate from said flotation treatment to con tinuouscentrifugal screening and recycling the matter passing through thecentrifugal screen to said flotation treatment.

'8. A method of dewatering and dressing very finegrained substanceswhich comprises in combination the steps of hydrophobising saidsubstances in a baflie plate mill, continuously centrifugally screeningthe hydrophobised substances, separating the matter passing through thecentrifugal screen into floatable particles and waste water, andreturning the floating fraction to the continuous centrifugal screening.

9. A method 01 dewatering and dressing very finegrained substances whichcomprises in combination the steps of hydrophobising and simultaneouslykneading said substances, continuously centrifugally screening thehydrophobised and kneaded substances, separating the matter passingthrough the centrifugal screen into floatable particles and waste water,and returning the floating fraction to the continuous centrifugalscreening.

10. A method of dewatering and dressing very finegrained substanceswhich comprises in combination the steps of hydrophobising andsimultaneously comminuting and homogenising said substances,continuously centrifugally screening the hydrophobised comminuted andhomogenised substances, separating the matter passing through thecentrifugal screen into floatable particles and waste water, andreturning the floating fraction to the continuous centrifugal screening.

11. A method of dewatering and dressing very finegrained substance whichcomprises in combination the steps of hydrophobising said substances inflotation plant, subjecting the concentrate from said combinedhydrophobising and flotation treatment to continuous centrifugalscreening and recycling the matter passing through the centrifugalscreen to said combined hydrophobising and flotation treatment.

Bureau of Mines Information Circular 7660, Convertol Process ofCoal-Slurry Treatment, by Thomas Fraser, April 1953. (4 pp. descr.; 3shts. drwg.)

1. A METHOD OF DEWATERING AND DRESSING VERY FINEGRAINED SUBSTANCES WHICHCOMPRISES IN COMBINATION THE STEPS OF HYDROPHOBISING SAID SUBSTANCES,CONTINUOUSLY CENTRIFUGALLY SCREENING THE HYDROPHOBISED SUBSTANCES,SEPARATING THE MATTER PASSING THROUGH THE CENTRIFUGAL SCREEN INTOFLOATABLE PARTICLES AND WASTER WATER, AND RETURNING THE FLOATINGFRACTION TO THE CONTINUOUS CENTRIFUGAL SCREENING.